| Electrical prospecting belong to the field of geophysical prospecting, and the DCresistivity method is an important method of electrical prospecting. Based on thedifference of the electrical properties of crustal geological, it research space distributionof the underground DC electric field under the artificial conditions of excitation current,to achieve the purpose of resources exploration and geological structures identification.Resistivity meter is a prospecting equipment based on DC resistivity method, includingmeasuring and control equipment, interpretation software and other ancillary equipmentsuch as electrodes, cables. In this paper, we design a earth resistivity measurementequipment which use a car battery as power supply, and the receiving system, the maincontrol system, together with the integration of power conversion system are integratedinto one single case.The system uses a switching power supply to take power from the battery to get thehigh-voltage DC signal, then use the H-bridge circuit to form a low frequency squarewave excitation signal with an interval of positive and negative, which is launched tothe underground through the launching electrode. Transmitter and receiver detect theemission current and receiving voltage signal, which collected by the ADC of the MCUC8051F060, then it transport the measurement results to a PC via RS-232C serialtransmission where and the data storage and processing are completed.Power system of the instrument use phase-shift control full-bridge ZVS PWM DC-DCconverter, boost circuits and vacuum circuit and linear regulator circuits to satisfy theneeds of400V high-voltage,±15V, and5V and3.3V DC Power Supply. Forphase-shifted full-bridge DC-DC converter, an integrated PWM controller UCC3895isused as the control core, and the boost circuit is implemented with a integrated DC-DCconverter MC34063. The instrument control system consists of two control layers,including a PC as upper computer and a MCU as lower computer. PC is responsible forsetting the measurement parameters and commands, and storage and processing ofmeasurement data, etc.; MCU C8051F060control the system and accomplish dataacquisition according to measurement command set by the upper computer, PC and MCU communicate through the RS-232C serial port.The system does a lot of work to improve the measurement accuracy, optimizeperformance and enhance stability. In power system, working power is divided intodigital and analog power supply, the digital and analog ground are separate, usingone-point grounded to reduce the interference to the high-precision analog circuitscaused by the digital circuits. In the control system, the MCU and power circuit isisolated by optocouplers, in order to reduce the interference to the high-precision analogcircuits caused by the power circuit. The excitation signal use a low frequency squarewave excitation signal with an interval of positive and negative, to measuring thenatural potential when the excitation current is zero and a group of opposite excitationsignal when the excitation current is not zero, then through the subsequent dataprocessing to eliminate the interference to the measurement result caused by earthnatural potential and the electrode polarization potential. In signal detection system,using the Hall current sensor to complete insulation measurement, in order tocircumvent the influence to the system under test when using the resistance samplingmethod; in receiving voltage detection system, using two programmable gaininstrumentation amplifier as preamplifier to suppress common mode interference andimprove signal to noise ratio, and adjust the measured signal amplification to the bestADC quantization interval, a50Hz notch filter circuit is used to suppress the powerfrequency interference. In addition, the system components use industrial gradeoperating temperature level, and the entire measurement system are integrated in asingle box, in order to improve the stability of the instrument when work in harshconditions. |
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